How do you achieve this horizontal stretch/distortion effect?

I've been lurking around the forums for quite some time, but normally, I've found the answers to my questions without having to ask myself. This time around, however, I could not seem to find an answer.

What I'm wondering is how the stretch/distortion effect in the video linked below is achieved? Any suggestions would be helpful!

at 1:06 and 3:51 (3:51 probably is the clearest example of what I'm looking for)